The present invention relates to excess flow valves used to prevent a flow of fluid through a port from exceeding a predetermined rate of flow.
Excess flow valves are, for example, commonly used in gas delivery systems, the excess flow valve being situated between the gas supply and the appliance using the gas, such that in the event of damage to or rupture of a pipe downstream of the excess flow valve resulting in an escape of gas, the valve will be operative to isolate the gas supply from the point of escape. In such circumstances, the valve responds to a substantial increase in flow rate.
These known excess flow valves rely upon a substantial increase in flow rate for triggering the valve to shut-off flow and so are normally located in high pressure systems.
Gas supply systems often include a bottle, or other high pressure gas source, connected to a pipe or similar means which is capable of delivering gas at high pressure to a regulator which supplies the gas at a low pressure to a low pressure part of the gas delivery system. Typically the pressure on the high pressure side varies between 10 to 250 psi (0.69 bar to 17.25 bar) and the pressure on the low pressure side varies between 28-50 millibar.
Provision of the valve on the low pressure side of the regulator does confer a number of advantages over well known systems located on the high pressure side.
It is also known to use mains gas appliances such as barbecues and lamps for outdoor use, and such as cookers and heaters for indoor use where the unintentional disconnection of apparatus or break in a gas pipe can be dangerous. Gas supplied in this context is at a relatively low pressure, typically 20 millibar, and thus high pressure excess flow valves are unsuitable in this application.
The present invention is directed toward the provision of an excess flow valve capable of being responsive to a predetermined increase in flow rate at low flow rate values and so can be used on the low pressure side of a gas delivery system. Such a valve may also be used in high pressure systems to respond to increase in flow above a predetermined low flow rate. Throughout the specification, it is intended that reference to the action of "shut-off" of the gas flow includes the situation where the gas flow is not completely shut-off, but substantially reduced to a small flow, whether said small flow is of a predetermined quantity or not.
An advantage of the present invention is that in a liquefied gas system the same excess flow valve may be used on the low pressure side of a delivery system for different gases which have different pressures on the high pressure side of the regulator and would thus otherwise each require high pressure excess flow valves of a different specification. In the particular example of butane and propane gas delivery systems, the regulated pressure on the low pressure side of the system is normally required to be essentially the same, whereas on the high pressure side of the regulator, the pressure will be substantially different for the two gases, thus requiring different types of high pressure excess flow valves.
The pressure in a gas bottle is also subject to fall as the quantity of gag remaining falls, and it is known that this can cause a substantial change in the flow rate permitted by an excess flow valve situated on the high pressure side of the regulator. The present invention also overcomes this problem.
It is further known that variations in temperature of the gas bottle can cause considerable changes in pressure within the bottle, with consequent changes to the flow rate permitted by an excess flow valve situated on the high pressure side of the regulator. The present invention also overcomes this problem.
It is further known that once the gas pressure in the bottle has fallen below a certain level, some excess flow valves situated on the high pressure side of the regulator may not function at all. In one particular example, when a system is used in conjunction with an Automatic Changeover device, the pressure may be allowed to fall to a level below which the excess flow valve will operate. The present invention also overcomes this problem.
A further known disadvantage of excess flow valves situated on the high pressure side of the regulator is that upon first allowing gas into an empty pipe, there is a surge of gas through the system which commonly causes the excess flow valve to operate, preventing further filling of the gas pipe. In this instance, the excess flow valve may need to be reset several times, or manually over-ridden during this initial pipe filling operation. If the excess flow valve is situated on the low pressure side of the regulator, this surge of gas may not occur, or may be very much reduced. Multiple resetting of the valve is thus eliminated or substantially reduced.
A difficulty with situating the excess flow valve on the low pressure side of the delivery system is that the difference between "normal" flow rate and excess flow rate may be small, and a high back pressure from a device downstream of the valve is not expected. The present invention is capable of distinguishing between these small differences of normal flow and excess flow.
A further advantage of the present invention is that it may be utilized in standard gas main pipework where gas is delivered at low pressure and low flow rates and can respond to a small predetermined rise in flow rate in order to shut off supply.